In ceramic materials, and specifically in ceramic floorings and coverings, porosity has a significant effect on mechanical and surface characteristics of materials.
In conventional ceramic materials, porosity values are minimum (close to zero) on the surface of the material due to the firing kiln process, which generates a closed-pore surface layer, whereas internal porosity is greater (2-5%). As a result, when the fired materials are subjected to any surface mechanical action (polishing), the surface layer thins out or disappears, and the internal porosity is exposed, affecting the properties of the material (reactivity, staining).
Porosity in ceramic materials or the like that are obtained by molding or pressing and subsequent firing depends on the characteristics of the starting materials and on the manufacturing process. Therefore, insufficient firing, the presence of organic material hindering combustion in the first stages of the heating process, or an irregular compaction or shaping process can substantially alter the porosity of the end product and therefore its properties.
Depending on the chemical composition of the product and on the shaping/firing process different types of porosity in ceramic materials are distinguished: “open porosity,” “closed porosity” or a combination of both. Open porosity is formed by interconnected pores leading to the formation of channels, whereas closed porosity relates to pores that are isolated from others in the ceramic matrix. Porosities in ceramic materials cover a broad range, from very open porosities to less open or closed porosities. In all cases, the porosity and the nature of said porosity are closely related to the characteristics and composition of the surrounding solid material.
When fired ceramic material is subjected to polishing, closed and open porosities are exposed on the surface of the ceramic material, affecting the properties of the material. The greater the porosity, the more reactivity or staining issues there are.
This porosity on the surface can be reduced by applying a layer of glaze that seals the surface. However, in the polishing step these protective surfaces are removed or erode the surface where they were applied, again leaving the closed porosity of the material exposed. In addition, given that the glaze is only present on the surface, the edge of the product clearly shows the lack of homogeneity of the product.
In unglazed materials, one of the most widely known being “through-body porcelain,” more attention is paid to the appearance of porosity than in glazed materials, but the porosity levels reached are not enough to fully protect the product against stainability with many chemical substances. For this reason, many manufacturers apply organic or inorganic sealing products serving as a surface protection. However, depending on how the surface will be used, sealers can break down, rendering them inefficient and leading to drawbacks of another type.
When closed porosity levels in the product are low enough, the surface reaction exposed by means of a mechanical action is very good. An outer surface having a low porosity is no longer affected by chemical reagents and the usual staining agents. However, the total lack of porosity in ceramic materials is undesirable because pores can absorbe produced stresses, impacts, etc., and therefore the lack thereof leads to brittleness and fracture issues in the material during processing. An example is glass, which lacks porosity but at the same time is very brittle.
In the case of manufacturing ceramic materials for use in kitchen countertops, “surface glazes” cannot be used to eliminate the porosity, because since it is a 3D product, the aesthetic integration between surface and edge would be broken. In addition, applying products known as “sealers” on the surface may violate regulations concerning contact with food. For these reasons, porosity levels must be reduced to a limit where stainability levels are insignificant by means controlling the chemical composition of the material and the method of manufacture.
In the case of inorganic ceramic products, their properties are virtually marked by the porosity of the material and by its chemical nature. These ceramic materials have a composition essentially based on three raw materials: clay minerals (clays and kaolins), sodium-potassium feldspars and feldspathic sand or quartz sand. In particular, feldspars act as melting during the firing step, being useful as a bridge for binding particles together and contributing to densification of the material.
Alves et al., Journal of European Ceramic Society, 2012, 32, 2095-2102 “Effect of feldspar particle size on the porous microstructure and stain resistance of polished porcelain tiles” describe the effect of feldspar particle size on the porous microstructure and the stain resistance of polished porcelain tiles.
Patent document WO2004/013066 describes stain-resistant vitrified porcelain products with very low or nil porosity as a result of particle packing, it specifically uses a five-component system to achieve a packing density close to 99%, and of the method of manufacture.
Patent document WO2014/009585 describes a new method for manufacturing high-performance ceramic materials, comprising a grinding step for grinding the raw materials to sizes of less than 200 micrometers, compacting and pressing at very high pressures to give rise to an ultra-compacted shaped product and a firing process. The obtained pieces have very good chemical and mechanical properties and can be made in large sizes.
Despite the different existing materials, there is still a need to provide compacted ceramic materials having large dimensions with very low internal porosity, and which can therefore be processed and polished without creating surface porosity. Furthermore, these materials must have high mechanical performance suitable for being used as building materials, such as outer covering and floor surfaces, for example, and in particular for manufacturing kitchen countertops.